DESCRIPTION: (applicant's abstract) Mononuclear pterin-containing molybdenum enzymes are found in eucaryotic and procaryotic organisms and function to catalyze simple oxo-atom transfer reactions (dimethylsulfoxide reductase and sulfite oxidase families) and the oxidative hydroxylation of aldehyde and heterocyclic substrates (xanthine oxidase family), The long term objectives of the proposed research are to understand the relationship between geometric and electronic structural features of the three main pterin-containing Mo enzyme families. This is important since it is the unique electronic structures of these active sites which make specific contributions to their reactivity thus defining their role in catalysis. Recent X-ray crystallographic studies have provided detailed structural information for enzymes from each main family defining the starting point for correlating geometric structure with enzyme function. These crystallographic results have allowed for the following hypotheses to be put forth: i) the degree of saturation of the pyranopterin dithiolene C=C bond, orientation of Spp orbitals and the degree of S...S bonding character are critical factors in determining how the cyanopterin dithiolene is capable of interacting with the redox active orbital (dxy) on the Mo center, ii) the oxidation state of the pyranopterin pyrazine ring modulates electron density within the dithiolene chelate, affecting the Spp-Mo dxy orbital interaction iii) the oxo donor ligands dominate the Mo ligand field resulting in radically different bonding descriptions for dioxo ligation compared to monooxo ligation and iv) in the xanthine oxidase enzyme family the presence of a Mo=S group cis to a Mo=O group substantially affects the electronic structure of the terminal sulfido , modulating its effective electro/nucleophilicity. The proposed research relies heavily on detailed spectroscopic studies of relevant model compounds and pterin-containing Mo enzymes. The research plan will employ a combined spectroscopic approach utilizing low temperature electronic absorption, magnetic circular dichroism and resonance Raman spectroscopies to correlate spectroscopic features with geometric structure and eventually determine electronic structure contributions to reactivity. The results of the proposed research should lead to developing detailed mechanistic insight into gated electron transfer, oxygen atom transfer and the activation of C-H bonds for oxidative hydroxylation by [MoOS] units.